The present invention relates generally to the field of plasma arc keyhole welding and in particular to a new and useful process for using a flux during plasma arc keyhole welding to provide improved welds.
Fluxing agents are known for adding to molten weld puddles in welding processes for the purpose of protecting the molten metal from atmospheric contamination by oxygen, nitrogen or hydrogen, or removing contaminants such as sulfur and-oxygen from the molten weld puddle. Welding processes for which the use of fluxing agents are known are submerged arc welding, gas metal arc welding, shielded metal arc welding, electroslag welding, and to some degree, gas tungsten arc welding. However, the inventors are not aware of any prior use of fluxing agents with plasma arc welding processes. Further, there is no documented use of fluxing agents in plasma keyhole welding for the purpose of reduction of sagging tendencies where molten metal is pulled out of the joint by gravity.
Plasma arc keyhole welding produces a hole through a workpiece having molten metal sides. As the welding arc is moved along the joint, the molten metal-sided keyhole progresses along the weld seam. The solidifying metal at the trailing edge of the weld creates the final weld. Initiating a plasma arc keyhole can lead to instability of the molten weld metal as the flow of plasma gas bores into the workpiece and exits the root side of the weld. The amount of instability can be affected by a number of factors including material thickness, composition and joint design.
In particular, in the case of plasma arc welding, materials over 0.250 inches thick often experience significant instability problems.
Molten metal drop-through is a common problem experienced during plasma arc keyhole welding. Gravity causes the molten metal to sag through the joint, and in some cases, to fall out of the joint.
Production experience has shown that approximately 3% of start areas in welds on nickel-based materials exhibited sagging when the keyhole was initiated. Most applications of plasma arc keyhole welding recommend the use of a disposable weld tab for arc initiation due to the known instability problem. But weld tabs cannot be used in all applications.
It is an object of the present invention to reduce molten metal drop-through during plasma arc keyhole welding using a fluxing agent.
It is a further object of the invention to provide specific fluxing agents for use during plasma arc keyhole welding to provide improved weld stability and quality.
Accordingly, a fluxing agent is added to a plasma arc keyhole weld. In the case of nickel-based alloys and stainless steels; the fluxing agent is preferably a titanium nitride-containing fluxing agent. The fluxing agent is added to the surface of the weld joint using physical vapor deposition or mechanically painting a flux slurry onto the weld material surface. Alternatively, the flux may be added to the base weld metal during manufacture of the metal alloy or as a powder to the molten metal puddle at the start of the welding operation.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying descriptive matter in which a preferred embodiment of the invention is illustrated.